The present invention relates to an image reproduction method and apparatus for converting an image pickup signal obtained from an image pickup element such as an image pickup tube, a CCD, or the like into an image signal that can be visibly output as, e.g., an NTSC-RGB signal.
Conventionally, in an image reproduction apparatus such as a TV camera using an image pickup element, e.g., a CCD, some of the image reproduction parameters are normally determined on the basis of image pickup data itself in image reproduction processing for obtaining image data from the image pickup data so as to usually obtain an image which gives the same impression in appearance at least or to faithfully reproduce an image, in spite of aging of the image pickup element and color filters or a change in illumination light source. The image reproduction parameters include, e.g., the color temperature, the reproduction luminance level, and the like, and are determined by correcting the color temperature or setting the reproduction luminance level.
In a TV camera of this type, so-called white balance adjustment is normally performed to correct the above-mentioned color temperature, so that an object which is to be seen in white is seen in white. In general, this correction is performed based on the picked-up image data. More specifically, an object to be seen in white is extracted from image data, and a white balance coefficient as one of the image reproduction parameters is determined from the extracted object. Normally, in the white balance adjustment, a plurality of color component signals constituting a video signal which is output from an image pickup element, are corrected by using the white balance coefficient, with gains assigned to each of these signals. This correction makes it possible to adjust the output levels of the respective color component signals constituting a video signal of an object which is to be seen in white, to become equal to each other.
For setting the reproduction luminance level, the luminance distribution is calculated based on image pickup data, an optimal reproduction luminance level (range) is set, and the parameters are adjusted to obtain a reproduced image within the set range, thus reproducing an image.
Such a method of adjusting the color tone of an image using the white balance adjustment is effective when an object to be seen in white (an object to be photographed) is sufficiently present in a video signal obtained from, e.g., an image pickup element.
However, in a specific situation, no object to be seen in white (an object to be photographed) may be present in an image pickup signal or even an object to be seen in white may be present, is is sometimes very small.
In this situation, it is impossible to adjust the color tone by the white balance adjustment. In general, in such a situation, the video signal of the entire image pickup data is averaged in units of color components to calculate average values, and the white balance adjustment is performed using the average values. However, with this method, a color represented by the calculated average values is not always white (the color of a light source), and white balance adjustment cannot be precisely performed.
As described above, when image reproduction processing is performed to obtain an optimal reproduced image, a condition for determining parameters is generally not satisfied if the image reproduction parameters are determined based on image pickup data. More specifically, sufficient information required for determining a specific parameter such as light source information in white balance processing, cannot sometimes be obtained, and the parameters cannot be precisely determined.
In a setting of the reproduction luminance level, if the reproduction parameters of luminance levels are determined in units of individual image pickup data, a correlation or the like in terms of luminance among a plurality of image pickup data to be compared, is lost, upon execution of image comparison, image synthesis processing, or the like. This makes a comparison between reproduced images difficult, or the luminance levels lose their continuity and become unnatural in a synthesized image. For example, the above-mentioned drawbacks are encountered when an object to be picked up as one frame is divisionally picked up, in terms of the photographing area, and a plurality of image pickup data are obtained.
More specifically, as for a reproduced image obtained by determining the image reproduction parameters based on image information, since images based on the image reproduction parameters corresponding to individual image pickup data are independently obtained, a reproduced image cannot be obtained by extracting information among images by comparing and analyzing a plurality of images, like in a case wherein physical property information or the like is obtained based on luminance information. For this reason, for example, when the reflectance spatially gradually changes, the luminance distribution ranges obtained in units of individual image pickup data are different from each other. When images are reproduced by optimizing such ranges in units of image pickup data, images are reproduced in which a correlation in terms of luminance, which is originally present in each of the image pickup areas, is lost. Therefore, if one image is obtained by synthesizing images which are obtained by picking up the respective image pickup areas, an unnaturally synthesized image in which a correlation in terms of luminance is lost, is undesirably obtained.